fix: Change cursor pagination to use After

[gmlewis]

Fixes: #3964.

@Not-Dhananjay-Mishra pointed out in #3964 that cursor-based pagination is not working, at least on the SecurityAdvisories.ListGlobalSecurityAdvisories endpoint, and was correct that using the After field instead of the Cursor field solved the problem.

This has been tested now and without using a throttling transport, I quickly got rate-limited. 😅

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.52%. Comparing base (de4dfc3) to head (6f4b31f).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3994   +/-   ##
=======================================
  Coverage   93.52%   93.52%           
=======================================
  Files         207      207           
  Lines       17591    17591           
=======================================
  Hits        16452    16452           
  Misses        938      938           
  Partials      201      201           

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[gmlewis]

Thank you, @alexandear! I added the integration test.
Merging.